This invention relates to an apparatus for measuring SIR. More particularly, the invention relates to an SIR measurement apparatus for measuring S/N ratio, which is the ratio of a desired signal power to noise power, or S/I (signal interference) ratio, which is the ratio of desired signal power to interference signal power.
In order to control and monitor communication quality in wireless communication, it is required that the signal-to-noise ratio S/N or signal-to-interference ratio S/I of a received signal be measured in the receiver. In a signal which uses CDMA (Code Division Multiple Access) now being considered as the next generation of communications technique, a characterizing feature is that interference power decides the capacity of the system. For this reason, a CDMA system employs a closed-loop transmission power technique through which the S/I ratio is held constant. In order to carry out such transmission power control, measuring S/N ratio or S/I ratio is essential.
FIG. 16 is a diagram useful in describing the necessity of transmission power control. Shown in FIG. 16 are a base station (BS) 1 and mobile stations (MS.sub.1 .about.MS.sub.n) 2.sub.1 .about.2.sub.n. Communication is carried out by CDMA. CDMA is a multiple access method using spread-spectrum communication, in which the base station 1 spread-spectrum modulates transmission information of a plurality of channels or users by codes (pseudorandom noise, or PN, sequences) that differ from one another, and transmits the spread-spectrum modulated signals upon multiplexing the same. Each of the mobile stations 2.sub.1 .about.2.sub.n de-spreads the received code-multiplexed signal using its own code (e.g. PN sequence) assigned at the time of communication and demodulates the transmission information addressed to the station. Each mobile station also spread-spectrum modulates transmission information by PN and sends the information to the base station 1. In mobile radio using CDMA, the signals from the mobile stations 2.sub.1 .about.2.sub.n reach the base station 1 while being superposed in time. The signal strength (power) which reaches the base station 1 from each of the mobile stations 2.sub.1 .about.2.sub.n differs depending upon disparities in distance, communication environment of transmission links and transmission power. As seen from a first mobile station, the signal output from another mobile station is interference. The first mobile station will be incapable of communicating if the strength of the signal from the other mobile station becomes too large. It is necessary, therefore, to hold constant the S/I ratio, namely the ratio of signal power which reaches the base station 1 from prescribed mobile stations 2.sub.1 .about.2.sub.n, to the power of interference waves, which include thermal noise. This makes it necessary to control transmission power.
FIG. 17 shows the flow of processing for transmission power control using a closed-loop system. The base station BS measures the S/I ratio, namely the ratio of received signal power from a mobile station MSi (i=1, 2, . . . n) to interference signal power (step 111) and notifies the mobile station MSi of the measured S/I ratio using an outgoing signal (step 112). The mobile station MSi determines whether the base station BSi has notified it of the S/I ratio (step 121), decides transmission power based upon the S/I ratio of which it has been notified and transmits its signal at this transmission power (step 122).
FIG. 18 is a block diagram useful in describing the position at which a conventional SIR measurement apparatus 4 is disposed relative to a receiver 3. It should be noted that a transmitter distributes serial data alternately one bit at a time to split the data into two sequences, namely in-phase component data and quadrature component data. The data in each of the two sequences is spread-spectrum modulated by being multiplied by PN, quadrature phase-shift keying (QPSK) modulation is applied to the spread-spectrum modulated signals of the I and Q components and the resulting signal is transmitted. The receiver 3 has an antenna 3a, a broadband band-pass filter 3b which passes only the necessary frequency band, a quadrature demodulator (QDET) 3c which demodulates spread-spectrum modulated signals V.sub.I, V.sub.Q, de-spreading circuits 3d.sub.I, 3d.sub.Q to which the spread-spectrum modulated signals V.sub.I, V.sub.Q of the I and Q components are applied as inputs and which output data D.sub.I, D.sub.Q of the I and Q components, and a data demodulator 3e for applying reverse-rotation processing to the data D.sub.I, D.sub.Q in an amount equivalent to the phase rotation produced by transmission, deciding the level of the results of rotation processing and outputting the reproduced data.
The de-spreading circuits 3d.sub.I, 3d.sub.Q respectively include multipliers 5.sub.I, 5.sub.Q for multiplying the spread-spectrum modulated signals V.sub.I, V.sub.Q by PN sequences C.sub.I, C.sub.Q identical to those on the transmitting side, and integrators 6.sub.I, 6.sub.Q for integrating the multiplier output signals over one symbol interval and successively outputting the results of integration, namely the I- and Q-component data D.sub.I, D.sub.Q. When the spread-spectrum modulated signals V.sub.I, V.sub.Q of the I and Q components are expressed on an I-jQ complex plane, the result is as shown in FIG. 19, in which a resultant vector V is the vector of the spread-spectrum modulated signal on the I-jQ complex plane.
FIG. 20 is a block diagram illustrating the construction of the SIR measurement apparatus 4. Shown at 3d is a de-spreading circuit (which corresponds to the de-spreading circuits 3d.sub.I, 3d.sub.Q of FIG. 18). The SIR measurement apparatus 4 includes a power arithmetic unit 4a for calculating power P of a spread-spectrum modulated signal, before the signal is de-spread, in accordance with the equation EQU P=V.sub.I.sup.2 +V.sub.Q.sup.2
averaging arithmetic unit 4b for calculating the average value of power covering N symbols, an interference power calculating unit 4c for multiplying the average power by 1/PG (where PG represents the spreading ratio) to calculate interference power I, a desired signal power arithmetic unit 4d for calculating desired signal power Pd after de-spreading in accordance with the equation EQU Pd=D.sub.I.sup.2 +D.sub.Q.sup.2
an averaging arithmetic unit 4e for calculating the average value S of desired signal power over N symbols, and an SIR arithmetic unit 4f for calculating the SIR from the desired signal power S and interference signal power I in accordance with the equation EQU SIR=S/I
In spread-spectrum communication, the spreading circuit of the transmitter multiplies a digital signal by PN (a rectangular wave of random +1 and -1 levels) to spread-spectrum modulate the signal. The rate of change of the PN sequence (namely duration Tc of the rectangular wave) is set so as to change over at a very high rate in comparison with symbol changeover speed (one bit interval T of the data) to be modulated thereby. That is, T&gt;&gt;Tc holds. The duration of T is referred to as the "bit duration", the duration of Tc is referred to as the "chip duration", and the ratio of T to Tc (i.e. T/Tc) is referred to as the spreading ratio, which is represented by PG. The band (=2/T) of the desired signal is spread by spread-spectrum modulation and becomes 2/Tc. That is, the band is spread by a factor of PG. As a result, the inputs to the receiver are a desired signal Sd, which is the result of spreading the band by a factor of PG by spread-spectrum modulation, and an interference signal Si, as shown in FIG. 21.
The power arithmetic unit 4a calculates the power of the signal that is the resultant of the desired signal Sd and interference signal Si, and the interference power calculating unit 4c multiplies the average power by 1/PG to calculate the interference signal power I (the portion hatched from lower left to upper right in FIG. 21) whose bandwidth is the same as that of the desired signal. Meanwhile, the desired signal power arithmetic unit 4d and averaging arithmetic unit 4e calculate the average value S of the desired signal power after de-spreading, and the SIR arithmetic unit 4f calculates the SIR by the operation S/I and outputs the signal representing SIR.
FIG. 22 is a block diagram illustrating another example of the construction of as SIR measurement apparatus 5 according to the prior art. The receiver 3 has the same construction as the receiver shown in FIG. 18 and identical components are designated by like reference characters.
The SIR measurement apparatus 5 includes a signal-point position altering unit 5a which, as shown in FIG. 23A, converts a received signal point D (whose I and Q components are D.sub.I and D.sub.Q, respectively) in the I-jQ plane to a point in the first quadrant of the plane. More specifically, the signal-point position altering unit 5a takes the absolute values of the I component (in-phase component) D.sub.I and Q component (quadrature component) D.sub.Q of the received signal D to convert the received signal to a signal in the first quadrant of the I-jQ complex plane. The SIR apparatus 5 further includes an averaging arithmetic unit 5b for calculating the average value m of N symbols of the received signal, a desired signal power arithmetic unit 5.sub.c for calculating m.sup.2 (power S of the desired signal) by squaring the I and Q components of the average value m and summing the squares, and an ideal position vector output unit (pilot) 5d for outputting an ideal signal-point position vector of a pilot symbol. The unit 5d senses a pilot symbol that has been inserted in a data frame and outputs I and Q components (vector Dip) conforming to the ideal signal point (already known) of the pilot symbol, as shown in FIG. 23B. The SIR apparatus 5 further includes an error vector arithmetic unit 5e for calculating an error vector D.sub.ERR between an actual position vector D.sub.AP of a pilot symbol and the ideal point position vector D.sub.IP, an error power arithmetic unit 5f for calculating variance .sigma..sup.2 (power of the error vector) of received power by calculating the square of each axial component of the error vector, an averaging arithmetic unit 5g for calculating the average value of the error power and outputting interference signal power I, and a SIR arithmetic unit 5h for calculating the SIR from the desired signal power S and interference signal power I in accordance with the equation EQU SIR=S/I
If we let xi (i=1, 2, . . . N) represent an input signal which contains a desired signal and interference, then the average value m of the input signals will be expressed by the following equation: EQU m=(1/N).multidot..SIGMA.xi (i=1, 2, . . . N)
and the result of squaring the average value m is the desired signal power. On the other hand, the average value .sigma..sup.2, which is the result of squaring the difference between the input signal and the average value, is the interference signal power. This is expressed as follows: EQU .sigma..sup.2 =(1/N).multidot..SIGMA.(xi-m).sup.2 (i=1, 2, . . . N)
The signal-point position altering unit 5a, averaging arithmetic unit 5b and desired signal power arithmetic unit 5c squares the average value m of the input signals to obtain the desired signal power S. On the other hand, the ideal position vector output unit 5d, error vector arithmetic unit 5e, error power arithmetic unit 5f and averaging arithmetic unit 5g obtain the interference signal power I. The operation S/I is performed by the SIR arithmetic unit 5h, which outputs the SIR.
With the SIR measurement method shown in FIG. 20, the average power of the signal which is the resultant of the desired signal Sd and interference signal Si is multiplied by 1/PG to calculate the interference power I (the portion hatched from lower left to upper right in FIG. 21). Consequently, the calculated interference signal power I includes the desired signal power (see the double-hatched portion in FIG. 21). This is a cause of measurement error. If the number of multiplex channels or number of users in CDMA is small, the proportion of the desired signal power contained in the interference signal power I increases and so does the SIR measurement error.
With the SIR measurement method shown in FIG. 22, it is required that the vector error between the received signal and the ideal signal be determined. As a result, it is required to execute pilot detection, calculation of vector error and squaring of the vector error on a per-symbol basis, and to perform averaging. A problem that arises is a complicated circuit arrangement and complicated arithmetic operations.